**4. Safety of organic materials**

The main concern associated with the use of organic materials is mainly related to the possible presence of unwanted components, such as microbial pathogens, heavy metals, organic pollutants, waste pharmaceuticals, and personal care products, which threaten public health when undertreated. For example, organic materials could contain pesticide residues if obtained from some crop residues or antibiotics used in the diets of breeding animals, if excrement is used.

### **4.1 Heavy metals**

The problem with regard to heavy metals is one of the most studied, and there is a vast literature dedicated to the subject. It is well known that concentrations of heavy metals above certain limits can lead to crop toxicity and may enter the food chain. The contents of MP in organic materials is very varied, since it depends on several factors, including the origin of the product, the feeding of livestock, etc. Rodriguez et al. [14] report the following total concentrations of heavy metals in cattle compost (in ppm): As 2.0 (−0.3), Cd 0.21 (−0.06), Hg <0.01, and Pb 5.9 (−1.01) and, for bovine lombricompost (in ppm), As 3.6 (−0.90), Cd 0.46 (−0.10), Hg <0.01, and Pb 16 (−2.60). For its part, Pane et al. [15] report the following heavy metal content in artichoke compost that was used to obtain nutrient solutions (78.0% artichoke, 20% woodchips, and 2% mature compost) (in ppm): Cd 0.38, Cr 20.69, Cu 21.01, Pb 13.45, Zn 13.45, and Zn 70.50, all below legal limits.

#### **4.2 Pathogens**

Depending on the source of the original material, the risks of contamination of unwanted organisms, such as pathogens, vary and are the highest in wastewater and excrement products.

Organic fertilizer production processes eliminate many pathogens as they include inactivation mechanisms such as very high temperatures, solar radiation, hydrolysis in strongly acidic or basic media, chemicals that affect pathogens, competition with other microorganisms, time, etc. (World Health Organization, 2018) [16]. If handled properly, composting can reduce pathogen levels [17]. In the inactivation of nonpathogenic *Escherichia coli*, pathogenic *E. coli* O157:H7, and *Salmonella* spp., several types of waste, such as animal manure and sewage sludge, have been reported during composting [18]. However, the persistence of *Listeria* spp., *Salmonella* spp., and nonpathogenic *E. coli* during composting [19] and the survival of *Salmonella* spp. and nonpathogenic *E. coli* in mature composts [20]. Most research on *E. coli* and *Salmonella* spp. have focused on manure or sewage sludge, but little attention has been paid to other substrates, such as green waste.

With regard to temperature, in many small composting units, degradation activity is limited by low temperature, well below 55°C. This is a very serious limitation when it comes to disinfection, since for many pathogens there is little or no reduction to temperatures below 50°C [16].

According to the US Environmental Protection Agency (US EPA) standard, Class A compost should not exceed the maximum *Salmonella* spp. limits (less than 3 most likely numbers [NMP]/4 g) or thermotolerant coliforms (less than 1000 NMP/g). The final amounts of bacteria, biological and viral, depend on the type of treatment used.

The current trend adopted in this field is to establish rigid rules that control the production process as well as to establish transport, packaging, and storage standards rather than setting pathogen limits on final products. For example, to acquire the characteristics necessary to be used in agriculture, sludge must undergo an additional disinfection process that ensures the reduction of the density of pathogens [16].

With regard to the risks of pathogens in organic fertilizers, it can be said that hazards can be excluded when production is industrialized, and this includes several disinfection procedures (pasteurization, drying, chemical media, etc.).

In addition, more or less stabilized organic substances, if poorly preserved and stored, can serve as excellent substrates for pathogens and become carriers of infections [21].

In the use of organic fertilizers, it is necessary to apply the precautionary principle, with the adoption of protective measures if there are suspicions that the products present a risk to public health or the environment. On the other hand, the danger of organic fertilizers and their amendments is certainly related to the end use of products.

Many organic compounds persist for long periods in soil, subsoil, aquifers, surface water, and aquatic sediments. These compounds, which can be of low or high molecular weight and that resist biodegradation, are known as recalcitrant. Many pesticides, mainly herbicides, have this characteristic [22].

Composting has been widely used for the remediation of organic pollutants as it, with adequate aeration, water, C-to-N ratio, and duration, accelerates their destruction [23]. The degradation of pesticides during composting depends on the pesticide and the substrate on which it is co-composted [24]. Strom [25] reported on the breakdown of organophosphorous pesticides and carbamates during composting. However, organochlorinated insecticides are resistant to degradation (Buyuksonmez et al., 1999). Differences in degradation may be related to inherent differences in the biological metabolism of the compound but may also be related to the composting process. Short-term composting (<60 days), which consists largely of the thermophilic phase, without adequate curing (mesophilic phase), may not be sufficient for the degradation of pesticides [26].

**83**

2 mmol L<sup>−</sup><sup>1</sup>

*Nutritive Solutions Formulated from Organic Fertilizers DOI: http://dx.doi.org/10.5772/intechopen.89955*

**5.1 Humic acids**

**5. Humic acids, microorganisms, and hormones in organic materials**

Organic materials, in addition to being a source of mineral elements (macronutrients and micronutrients), also provide the SN with other inseparable substances, among which are the microorganisms, humic acids (HA), and phytohormones.

Humic substances (HS) are the last substances resulting from chemical, biological, and physical transformations of plant and animal matter. The main compounds resulting from this transformation are humic acids, fulvic acids, and humines. Within these substances, humic acids, compounds soluble in alkaline solution and insoluble in acid solution and having a higher molecular weight, are the most important components [27, 28]. These substances, for their characteristics and

HS are mineral compounds, among them essential elements for plants, mainly carbon, oxygen, hydrogen, nitrogen, sulfur, phosphorus (P), iron, copper, zinc and boron, in addition to functional groups among which stand out aromatic, aliphatic, carboxylic, and phenolic compounds (from [30–32]). HS are composed of hydrophobic fractions composed of aliphatic and aromatic compounds, while in another fraction, hydrophilic is composed of irregular humic fractions. These compounds,

effects on plants, have been considered as biostimulants [29].

within the plant, of metal ions [37].

addition of 50 mg L<sup>−</sup><sup>1</sup>

Haghighi and Teixeira [38] added 25 mg L<sup>−</sup><sup>1</sup>

Jannin et al. (2012) used 100 mg L<sup>−</sup><sup>1</sup>

for their physicochemical characteristics, cause various effects on plants.

Among the metabolic processes that contribute to promote the growth and development of plants is the stimulation of the activity of key enzymes for the absorption and distribution of nutrients [33, 34]. The interaction of humic substances with proteins and lipids of the cell membrane improves the absorption of nutrition [35]. Mora et al. [36] mention that the presence of AH stimulated the activation of the H+-ATPase pump which led to a better distribution of NO3

the root to the leaves. HSs can form latent complexes with metal ions, contributing to increased availability for root absorption as well as improving the distribution,

forest soil moistified monthly to the nutrient solution used in the cultivation of tomato grown in perlite/vermiculite substrate. These HS were composed of 0.57% nitrogen, 0.03% phosphorus, and 4.5% potassium, with a pH of 4.5. Basically the

variables to the low concentrations of HS evaluated in the experiment.

plants, increasing by 19% yield, 29% protein, 436% photosynthesis in growth stage, and 34% in fruiting stage. Other variables such as nitrate content, sugar content, and acidity in addition to antioxidant enzymes and chlorophyll were not affected by the presence of HS. These authors attributed the null effect on the abovementioned

formulation of Hoagland and Arnon nutrient solution (1950), for the cultivation of canola in floating root system. This material contained mainly 125, 40, 14, 9, and

in addition to very low amounts of cytokinins such as zeatin, isopentenyladenine, and isopentenyladenosine. The plants were evaluated at days 1, 3, and 30 after the start of treatment, wherein the most significant effects were found at 30 days. The dry root weight was increased by 88%, while the total dry weight of the plant was

of potassium, sulfur, calcium (Ca), iron, and phosphorus, respectively,

There are various materials from which HS is obtained, which have been used in different crops in the hydroponic system. These substances have shown significant effects on these plants, improving growth and nutritional condition, mainly.

and 50 mg L<sup>−</sup><sup>1</sup>

HS extracted from black peat for the

of HS was the treatment that provoked the greatest effect in

<sup>−</sup> from

of HS extracted from
